Method and system for transmitting terahertz pulses

ABSTRACT

Systems for THz transmission using new types of THz waveguides with low loss, negligible group velocity dispersion and structural simplicity are described herein. The THz system incorporates the use of a waveguide with two or more substantially parallel conductive elements which may enable many new THz sensing applications. It is now possible to direct the THz pulse inside of containers or around corners, where line-of-sight optics are not practical. Moreover, the systems allow use of either radially polarized or linearly polarized THz antennas. The disclosed systems are compatible with existing terahertz generation and detection techniques.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication Ser. No. 60/807,380 filed Jul. 14, 2006, herein incorporatedby reference in its entirety for all purposes.

TECHNICAL FIELD OF THE INVENTION

This invention provides a method and system for guiding terahertz pulsesover practical distances with low loss and low dispersion.

BACKGROUND OF THE INVENTION

Rapid advances in laser technology have enabled various techniques forthe generation and detection of electromagnetic radiation in theterahertz region (spanning from ˜100 GHz to ˜10 THz, or wavelengthsbetween ˜30 μm and ˜3 mm). As a result, numerous uses of terahertzradiation have been explored, including trace gas detection, medicaldiagnosis, security screening, and defect analysis in complex materialssuch as space shuttle tiles. Many of these studies have relied onterahertz time-domain spectroscopy, a technique for generatingsub-picosecond pulses with spectral content spanning much of the THzband.

However, progress is limited by the overwhelming reliance on free-spacetransport of the terahertz beam, using bulk optical components. In manyreal-world situations, the sample or region to be studied may not bereadily accessible to a line-of-sight beam. Hence, common devices thatoperate at other wavelengths, such as optical fiber-based sensors ormedical endoscopes rely on the guided wave delivery of light to theremote sensing location. In addition, while THz waves can be transmittedsimply by free space propagation, free space propagation requires bulkoptical components, which are difficult to align.

Thus, in order to expand the usefulness of THz radiation, it isdesirable to provide optimized guided wave devices that operate at THzfrequencies. The development of practical THz waveguides woulddramatically expand the application of THz-TDS in areas such as gassensing and nanometer thin-film measurements.

Heretofore, the development of THz waveguides has been hindered by thematerial properties and the application requirements in this spectralrange. On the one hand, the characteristics of materials at THzfrequencies make it extremely difficult to build a fiber to guide THzbeams over a long distance. The most transparent materials for thisrange are crystalline (e.g., high resistivity silicon), and thus arecostly, fragile, and challenging to form into specific geometries forwaveguide configurations. Other materials, such as low-loss polymers orglasses, are more malleable but exhibit prohibitively high absorptionlosses for propagation distances of more than a few centimeters. Forthis reason, THz waveguides generally must rely on propagation in air,rather than via dielectric confinement as in an optical fiber.

On the other hand, many THz applications rely on the use of broadbandpulses for time-domain analysis and spectroscopic applications. To avoidpulse reshaping during propagation, low dispersion is required. But formany conventional metal waveguides (e.g., metal tubes), pulse reshapingin propagation is difficult to avoid, due to the extreme dispersion nearthe waveguide cutoff frequencies. Furthermore, finite conductivity ofmetals can lead to considerable losses in the wave propagation.

Great efforts have been devoted to finding useful THz waveguides withinthe last few years, and various guides with quasi-optical coupling havebeen demonstrated. Most of these THz waveguides have been based onconventional guiding structures, such as metal tubes, plastic ribbons,or dielectric fibers. There have also been reports on the application ofthe latest technology of photonic crystal fibers to THz radiation. Inall of these cases, the utility for transport of THz pulses is limitedby group velocity dispersion of the guided waves.

The most promising studies have reported dispersionless propagation inparallel metal plate waveguides. Another type of dispersionlesswaveguide design is a ribbon waveguide, which is dispersionless andlow-loss. In the parallel plate design, the loss is attributable to twofactors: (1) lateral spreading due to the fact that the mode isunconfined in one of the two transverse dimensions, and (2) the finiteconductivity of the metal used to confine the mode, which in this caseresults in a reported attenuation of ˜80 dB/m. Furthermore, theattenuation is still unacceptably high, due in large part to the finiteconductivity of the metal plates. In addition, the cross-sectional areaof the waveguide is too large for many of the proposed THz applications,including in particular medical diagnostics. Consequently, there is aneed for a waveguide that is effective at transmitting radiation at THzfrequencies utilizing both linearly and radially polarized THz sourceswith low loss and dispersion while maintaining a low cross-sectionalarea for diagnostic and medical applications.

SUMMARY OF THE INVENTION

Systems for THz transmission using new types of THz waveguides with lowloss, negligible group velocity dispersion and structural simplicity aredescribed herein. The waveguides enable many new THz sensingapplications. It is now possible to direct the THz pulse inside ofcontainers or around corners, where line-of-sight optics are notpractical. Moreover, the systems allow use of either radially polarizedor linearly polarized THz antennas. The disclosed systems are compatiblewith existing terahertz generation and detection techniques. In someembodiments, a THz system includes the use of a coaxial antenna and acoaxial or uniaxial waveguide. In preferred embodiments, the THz systemincorporates the use of a waveguide with two or more substantiallyparallel conductive elements. Further advantages and features of thesystems and methods are described in more detail below.

In an embodiment, a terahertz (THz) transmitting system comprises anantenna capable of generating a beam of THz radiation. The system alsocomprises a waveguide aligned with the beam. The waveguide has at leasta first and a second conductive elongate element in which the first andsecond conductive elongate elements are substantially parallel.

In another embodiment, a method for transmitting a terahertz signalcomprises aligning an antenna capable of generating a beam of THzradiation with a waveguide comprising at least a first and secondconductive element. The method further comprises generating a beam ofTHz radiation by focusing a laser at said antenna. In addition, themethod comprises collimating and focusing said beam between the firstand second conductive element of said waveguide. The method alsocomprises transmitting said beam along the length of said waveguide bypropagating said beam between the first and second conductive element.

In yet another embodiment, a terahertz interferometer comprises a THzemitter that generates THz radiation. The interferometer also comprisesa primary waveguide aligned with said emitter. The primary waveguidecomprises at least a first and second conductive element where the atleast first and second conductive element diverge to form a samplewaveguide and a reference waveguide. Additionally, the interferometercomprises sample chamber where the sample waveguide passes through saidsample chamber. The sample waveguide and said reference waveguide rejointo form a secondary waveguide. Moreover, the interferometer comprises adetector capable of receiving THz radiation. The detector is alignedwith the secondary waveguide.

Embodiments of a THz transmitting system further includes a metalwaveguide with very simple geometry that may be used to guide broadbandTHz pulses with outstanding performance, including low loss andnegligible group velocity dispersion. The guided propagation of THzpulses on a metal wire behaves similarly to the transverseelectromagnetic (TEM) mode of a conventional coaxial waveguide. Sincethe exposed surface area of a wire is much smaller than that of anypreviously reported metal waveguide, the attenuation due to conductivitylosses is extremely low for this configuration. The efficacy andstructural simplicity of the disclosed waveguide present greatadvantages in the manipulation of guided THz radiation.

The foregoing has outlined rather broadly the features and technicaladvantages of embodiments of the invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter that form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other structures for carrying out thesame purposes of the invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of exemplary embodiments of the invention,reference will now be made to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a coaxial antenna constructed inaccordance with a first embodiment of the invention;

FIG. 2 is a schematic diagram of the coaxial antenna of FIG. 1 showingthe mode of generation of radiation;

FIG. 3 is a schematic diagram of a coaxial antenna coupled to a coaxialwaveguide;

FIG. 4 is a schematic diagram of an alternative embodiment of an antennacoupled to a coaxial waveguide;

FIG. 5 is a schematic diagram of an alternative embodiment of a coaxialwaveguide;

FIG. 6 is a schematic diagram of a coaxial antenna coupled to a uniaxialwaveguide;

FIG. 7 is a schematic diagram of one embodiment of a coaxial waveguideadapted to serve as a sensor;

FIG. 8 is a schematic diagram of one embodiment of a THz probe;

FIG. 9 is a schematic diagram of one embodiment of a THz endoscope;

FIG. 10 is a schematic diagram of one embodiment of a THz near-fieldscanning optical microscope;

FIG. 11 is a schematic diagram of a second embodiment of a THzwaveguide;

FIG. 12 is a plot of arrival time as a function of incident position fora series of time-domain THz pulses propagating along a bare metal probe(squares) and a probe wrapped with a 0.5 mm PVC insulation layer(triangles), as detected in a THz apertureless near-field scanningoptical microscopy system;

FIG. 13 is a schematic diagram of an experimental setup forcharacterization of a THz wire waveguide;

FIG. 14 is two plots showing the spatial profile of the propagating modeon a metal wire waveguide;

FIG. 15 is two plots illustrating the group velocity of propagating THzpulses on a metal wire waveguide;

FIG. 16 is a plot of the amplitude of a THz pulses as a function of thevertical displacement of the receiver, measured at different propagationdistances;

FIG. 17 is two plots illustrating the attenuation characteristic of theguided wave on a metal wire waveguide;

FIG. 18 is a plot illustrating THz wave propagation on a 21 cm long wirewaveguide with different bend radii R;

FIG. 19 is a schematic diagram of a simple Y-splitter structure,comprising a straight waveguide and a curved waveguide in contact witheach other; and

FIG. 20 is a plot of the THz waveforms detected at points A, B and C inFIG. 19;

FIG. 21 is a schematic diagram of an embodiment of a dual waveguide;

FIG. 22 is a schematic diagram of an embodiment of a coaxial dualwaveguide;

FIG. 23 is a cross-section of an embodiment of a coaxial dual waveguide;

FIG. 24 is a schematic diagram of a linear dipole antenna coupled to adual waveguide;

FIG. 25 illustrates an embodiment of a dual waveguide with a bend; and

FIG. 26 is a schematic diagram of an embodiment of a Mach-Zehnderinterferometer using a dual waveguide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Certain terms are used throughout the following description and claimsto refer to particular system components. In the following discussionand in the claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to . . . . ”

As used herein, a waveguide is intended to refer to a device designed toconfine and direct the propagation of electromagnetic waves.

Antenna

In order to efficiently couple radiation into a waveguide, it isdesirable to match the spatial pattern (the mode) of the incidentradiation to the mode of the waveguide. For a coaxial waveguide, thisrequires that the incident beam be radially polarized, since this is thecharacteristic of the waveguide mode with the lowest loss. Such apolarization profile is typically very difficult to generate forfree-space radiation, which is why coaxial waveguides have not oftenbeen used at high frequencies. Embodiments of the THz system usecomponents similar to those used in time-domain spectroscopy (TDS) togenerate a radial mode profile through the use of a novel, radiallysymmetric antenna.

A first antenna configuration is illustrated in FIG. 1. A first,relatively narrow lead 10 extends in a first direction and terminates ina small circular electrode 12. A second lead 20 extends in an oppositedirection and terminates in an annular electrode 22 that is concentricwith circular electrode 12. While electrodes 12, 22 are shown ascircular, they could alternatively be elliptical, oblong, polygonal, orany other non-circular shape. Likewise, lead 10 could extend farther, oreven across the diameter of annular electrode 22, so as to provideanother axis of symmetry in the antenna.

Electrodes 12, 22 are preferably formed on or adjacent to asemiconducting substrate so as to create a switch. A dc bias is placedacross the antenna by applying a voltage across leads 10, 20 and anultrashort pump-laser pulse (<100 femtosecond (fs)) is focused on theannular region 30 between electrodes 12, 22. The bias-laser pulsecombination allows electrons to rapidly jump the gap, so when thisregion is excited by the application of a fs laser pulse the resultingcurrent in the semiconductor produces a terahertz electromagnetic wave.This antenna operates in the same manner as a typical THz emitterantenna (commonly used in THz-TDS), except that the generated modepreserves the cylindrical symmetry of the antenna pattern and istherefore radially polarized, as illustrated by the arrows in FIG. 2.This radial mode is predicted to couple extremely efficiently (up to atleast ˜56%) into an optimized coaxial waveguide. Coaxial waveguides arediscussed in detail below.

In this context, optimization of the waveguide entails selecting thedimensions of the coaxial waveguide (i.e., the inner and outer radii) tomatch the size of the radially polarized mode. This selection ispreferably based on calculations of the mode matching for a given THzbeam spot size.

It is possible to use a similar antenna design for photoconductivedetection of the THz pulses emerging from a waveguide. This has theunique advantage that only the fundamental transverse electromagnetic(TEM) mode of the guide are detectable, so even if the launched wavewere propagating in a multi-mode regime, this would not lead tomeasurable group velocity dispersion but only to increased propagationlosses. Such considerations are important for any spectroscopic orimaging applications in which coaxial guides would be employed.

Coaxial Waveguide

As shown in FIG. 3, a coaxial waveguide 40 is preferably constructedwith an inner core 46 and an outer wall 48 constructed from a materialthat is substantially opaque to terahertz radiation such as copper orany other highly conductive material. In embodiments, core 46 may be awire and may be made of any suitable conductive material. Generally,core 46 is solid. However, it is contemplated that core 46 may be hollowor tubular. The annular region 44 between inner core 46 and outer wall48 is preferably either evacuated or filled with a substance that istransparent or substantially transparent to terahertz radiation, such asa gas. Further details relating to a preferred construction for thewaveguide are set out in detail below.

Still referring to FIG. 3, the radially polarized beam 32 generated byan antenna, such as that described above, can be coupled to a coaxialwaveguide in any suitable manner. In one embodiment, beam 32 may becollimated and focused on one end 42 of coaxial waveguide 40. Beam 32 ispreferably focused by a lens 50 such that a significant portion of itsenergy is incident on annular transmitting region 44. It is preferredthat beam 32 not be focused too tightly, as that will result in too muchof the energy being incident on core 46. Hence, it is preferred that thedegree of focus be optimized. This optimization can be performedexperimentally or mathematically.

In alternative embodiments, one or both of the electrodes of the antennamay be constructed on, attached directly to, or integral with the end ofthe waveguide. In these embodiments, circular electrode 12 may beattached to or formed by the end of the core of the waveguide and/orannular electrode 22 may be attached to or formed by the end of theouter wall of the waveguide. These embodiments avoid the need forcollimation and/or focusing while providing coupling at a very highefficiency.

In an exemplary embodiment, shown in FIG. 4, an annular electrode 20 isspanned by a web of semiconducting material 25 and a lead 10 is providedto the end of core 46, which serves as the inner electrode. The fs laserpulse is applied to semiconductor 25 while a voltage is applied acrossthe two electrodes. In other variations (not shown), the inner electroderather than the outer electrode may be separate from the waveguide andmay support a radially extending semiconducting web. Regardless ofwhether one, neither, or both of the electrodes is separate from thewaveguide, a sheet or web of semiconducting material extends normal tothe longitudinal axis of the waveguide and spans its cross-sectionalarea. Also, regardless of the configuration of the electrodes andwaveguide, it is not necessary that the dimensions of electrode 12coincide with those of the core of the waveguide or that the dimensionsof annular electrode 22 coincide with those of the outer wall of thewaveguide.

The emission properties of the antenna (the mode pattern and thefraction of the radiation coupled into the high-dielectric substrate)are preferably optimized with respect to the geometrical factors (innerand outer radii, line thicknesses, etc). This optimization can beperformed numerically, using commercially available finite elementanalysis software or any other suitable algorithms.

Fabrication techniques suitable for making the antennas orantenna/waveguide combinations are similar to the techniques used tomake conventional THz dipole antennas. Once fabricated, each antenna iscoupled to an femtosecond optical excitation beam, preferably usingfiber delivery (as has been done with some dipole antennas). Anadditional step may involve coupling or attaching these antennasdirectly to the input and output faces of the waveguide as describedabove, in order to achieve the highest possible levels of energycoupling into the guided mode.

Some embodiments of the waveguide include a core and outer wall, asdescribed above. In addition, because some embodiments include anevacuated or gas-filled annulus, it is preferred in those embodiments toprovide means for maintaining the core at the center of the waveguide.It is preferred that the inner conductor be positioned as precisely aspossible along the axis of the outer cylinder so as to maximize symmetryof the waveguide and efficiency of transmission. This can beaccomplished in any of several ways.

One approach, shown in phantom in FIG. 5, is to use a plurality ofcylindrical plugs 60, spaced apart along the length of the waveguidecylinder, with holes 62 drilled in their centers through which the innerconductor is inserted. If these holes are positioned accurately andplugs 60 are sized correctly, plugs 60 will hold inner conductor 46 inprecise coaxial alignment with outer wall 48. Plugs 60 are preferablycomposed of a material that has a low refractive index and is as nearlyinvisible to THz radiation as possible, such as rigid polystyrene foam,other polymeric foams, or, less preferably, non-foamed materials. Othermeans for supporting core 46 may be used, including but not limited tobraces, fins, and legs.

In preferred embodiments, the waveguide is substantially straight. Inalternative embodiments, the axis of the wave guide may include one ormore curves, however it is expected that a curvilinear waveguide willresult in some loss of efficiency, as some of the transmitted energy islost to modes that are not compatible with the waveguide. Morespecifically, curves in the waveguide tend to convert from the TEM modeto other modes. These other modes are not detected by the radialreceiver antenna, with the result that the detected energy is lower.Thus, bends introduce loss. It may be possible for the bend to alsointroduce dispersion, which is typically undesirable. Dispersion wouldinvolve coupling from the TEM mode into other modes, and then backagain.

Uniaxial Waveguide

In still another embodiment, a waveguide for THz radiation may compriseonly a conductive core 46. Because a significant portion of the lossesin a coaxial waveguide are the result of the finite conductivity of theouter wall, while only a portion of the losses are due to dispersion(diffractive spreading), a waveguide comprising only a core 46 maytransmit THz radiation sufficiently for some purposes, particularly overshort distances (less than a few meters). It is further expected thatsuch a uniaxial waveguide may include some degree of curvilinearity.Uniaxial waveguides in accordance with the invention may be constructedof any suitably conductive material, including but not limited to baremetal wire. In some embodiments, a bare metal wire with a thindielectric coating may be advantageous in confining the mode closer tothe surface of the wire. The material and diameter of the core may beoptimized according to the desired operating parameters such aswavelength and transmission distance but is preferably between 0.1 and30 mm in diameter, more preferably between 0.1 and 20 mm, and still morepreferably 0.5 and 10 mm in diameter The uniaxial waveguides may also beused in plural, in a THz sensor such as that shown in FIG. 8 anddescribed below.

A uniaxial waveguide, sometimes referred to herein as a “wirewaveguide,” may be coupled with a THz antenna in the manner(s) describedherein. For example, the center one of a pair of concentric electrodescan be affixed to the end of the waveguide or a radially polarized beamcan be focused on the end of the waveguide. The wire waveguide couldalso be coupled to the THz antenna through the use of a substrate lens,similar to the type of lens typically used in terahertz time-domainspectroscopy. Alternatively, a linearly polarized THz beam may befocused on a scattering object placed near the waveguide. If the beam issufficiently intense, enough scattered energy will be coupled into thewaveguide modes and transported along the waveguide to serve as adetectable signal. This latter technique is illustrated in FIG. 12.Finally, one could couple free-space radiation onto the waveguidethrough the use of a grating coupling scheme, in which a periodicmodulation of some property of the wire (e.g., the diameter) is used todiffract incident radiation along the wire axis, thus exciting theguided mode.

Dual or Parallel Waveguides

As shown in FIG. 21, in another embodiment, a waveguide for THzradiation comprises at least a first conductive elongate element 301 anda second conductive elongate element 302 to form a dual waveguide 300.Preferably, the conductive elements 301, 303 are substantially paralleland generally are equidistant from each other (i.e. uniformly spacedfrom each other along their entire lengths). As used herein, “parallel”means that the conductive elements 301, 303 lie in a common plane and donot intersect. Generally, dual waveguide 300 comprises bare conductiveelements. In other words, the conductive elements 301, 303 are notsurrounded by an outer wall or covering. Preferably, the elements arelong and thin wire-like elements. It is emphasized that the conductiveelongate elements are not plates. In an embodiment, each conductiveelement comprises a solid metal wire or lead. Alternatively, eachconductive element 301, 303 comprises a bundle of wires, filaments,leads, or fibers. In some embodiments, conductive elements may behollow. Moreover, first and second conductive element 301, 303 may havecylindrical, rectangular, hexagonal, triangular, or other crosssectional geometry. Although, dual waveguide 300 has been described thusfar as having two conductive elements, it is contemplated that in otherembodiments, the waveguide 300 may comprise more than two conductiveelements that are all substantially parallel to each other.

As shown in FIGS. 22-23, the dual elements 301, 303 may be surrounded byan outer wall 305 which is opaque to THz radiation to form a dualcoaxial waveguide 350. First and second conductive elements may befabricated from any high conductivity metal. The conductive elements arepreferably spaced such that first and second conductive element are nottouching and preferably such that the distance between them is constant.Generally, dual cores 301, 303 are no more than about 1 cm apart, morepreferably no more than 0.5 cm apart.

One advantage of the dual waveguide is that it does not require aradially polarized source although it can work with one. Therefore, thedual waveguide is capable of being used in conjunction with any type ofTHz transmitting antennae. In a preferred embodiment, the THz antenna isa conventional dipole THz antenna. Conventional THz dipole antennas emita linearly polarized beam of THz radiation as opposed to a radiallypolarized beam generated by a radially symmetric THz antenna as shown inFIG. 1. FIG. 24 illustrates an embodiment a terahertz transmittingsystem in which a dual waveguide is used with a conventional THz dipoleantenna 307. In general, linear dipole antenna 307 is preferably mountedon a semi-conducting substrate such as gallium arsenide or silicon onsapphire. As shown, linear THz dipole antenna 307 comprises first andsecond electrodes 322, 326. First and second electrodes 322, 326 aresubstantially parallel to each other. Additionally, first and secondelectrode 322, 326 each include an electrode extension 313, 315,respectively, which extend substantially perpendicular from first andsecond electrodes 301, 303. Inner electrodes 313, 315 extend toward eachother and are substantially aligned so that gap 312 is defined betweenthem. An ultrashort pump-laser pump is directed at the gap 312. When avoltage is applied across the gap and the laser is fired, a THz pulse isgenerated.

According to an embodiment, dipole THz antenna 307 is aligned with dualwaveguide 350 such that a beam 331 of THz radiation is collimated andfocused on the area 309 between the dual conductive elements. Inembodiments, dual waveguide 350 may be perpendicular to antenna 307.However, waveguide 350 may be aligned at any suitable angle which is notparallel to antenna 307. The THz beam is collimated using a collimator50 such as a hemispherical silicon dome as shown in FIG. 24. Collimator50 may be disposed between dipole THz antenna 307 and dual waveguide350. The linearly polarized beam 331 propagates down the dual waveguide350 between the first and second conductive element 301, 303. Withoutbeing limited by theory, it is believed that the dual waveguide hasreduced loss because of the decreased area of propagation. In herembodiments, dual waveguide 350 may be used with a radially symmetricantenna as disclosed above.

In yet another embodiment, the first and second conductive element 301,303 of dual waveguide 300 each includes a bend or curve 390 along theirrespective longitudinal axes while remaining substantially parallel toeach other as shown in FIG. 25. As with uniaxial waveguides, anotheradvantage of the dual waveguide 300 may be its ability to transmitlinearly or radially polarized THz pulses or signals around corners andangles other than a straight line. Alternatively, first and secondconductive element 301, 303 may comprise divergent curves or bends suchthat first and second conductive element 301, 303 form a split. As willbe described below, divergent conductive elements may be useful forinterferometry applications.

Referring now to FIG. 26, in one particular application involving a dualwaveguide, a Mach-Zehnder interferometer 400 may incorporate a dualwaveguide. The dual waveguide may comprise bare first and secondconductive elements or coaxial conductive elements. In an embodiment, aTHz beam is generated, as described above, with a conventional THzantenna 407 and collimator 411. However, the dual waveguide 402 includesa split between the first conductive element 401 and the secondconductive element 403. First conductive element 401 and secondconductive element 403 diverge from each other to form a samplewaveguide 418 and a reference waveguide 422. Without being limited bytheory, it is believed that the linearly polarized THz beam will splitinto two separate radially polarized beams when first conductive elementand second conductive element diverge and the split beam will follow thetwo paths laid out by the two conductive elements 401, 403.

Still referring to FIG. 25, sample waveguide 418 enters a sample chamber431 containing a sample compound of interest. The sample compound may bea liquid, a gas or a solid. Reference waveguide 422 remains exposed toambient air or ensconced in outer wall. Alternatively, referencewaveguide 422 may pass through a reference chamber containing areference material (not shown). Sample waveguide 418 exits samplechamber and rejoins reference waveguide 422 to re-form secondary dualwaveguide 413. It is envisioned that the two radially polarized beamswill merge and become a single linearly polarized beam at this point.Secondary dual waveguide 413 is aligned with a detector 471. Detector471 typically also comprises a conventional dipole THz antenna. Detector471 is capable of determining the intensity of the THz beam emitted fromthe end of dual waveguide 411.

As the THz beam traveling along sample waveguide 418 passes through amaterial with a refractive index the phase of the THz beam will beshifted. As the THz beam from the sample waveguide 418 rejoins the THzbeam from the reference beam, the sample beam will interfere with thereference beam. To the extent that the beams are out of phase, thisinterference will cause a corresponding decrease in the measuredintensity of the THz beam. In an embodiment, the THz interferometercould be used to detect minute differences in concentration between thereference chamber and the sample chamber.

Applications

Embodiments of the disclosed systems have many possible uses. First, theuse of guided terahertz pulses eliminates the need for free-spaceoptical components, which vastly simplifies the alignment of a terahertzspectrometer This makes the use of a terahertz system far more realisticfor many applications, particularly those for which sensitive alignmentis problematic. A good point of comparison in this case is the FourierTransform Infrared (FTIR) spectrometer, a device found in virtually allundergraduate analytical laboratories, at every university in the US. Ifa terahertz system were as easy to use as an FTIR, one could imaginethat it could also be used as a teaching tool in similar fashion.

In the disclosed THz systems, lateral spreading may be eliminatedbecause the mode is confined, but the finite conductivity of the metalremains as limitation on the efficiency of the waveguide. Thisconstraint could be minimized by using materials with very highconductivity such as copper or silver, or some other lower-conductivitymaterial coated with a thin layer of copper or silver. In the lattercase it is preferred that the thin layer be thicker than the skin depthof the high-conductivity material, which is roughly 1 micron. Inaddition, because the disclosed waveguides can have a smallcross-section and because of the simplicity of their design, thedisclosed waveguides are more compatible than other recentlydemonstrated THz waveguides with many envisioned applications, such asendoscopy.

Many terahertz imaging and sensing applications will requiretransmitting terahertz radiation to and receiving it from a sample thatis difficult to reach. Many of these applications require anendoscope-type configuration, to guide the terahertz waves to thesample, and then guide the reflected radiation back to a detector. Thedisclosed waveguides are perfectly suited for this purpose. For example,a waveguide may be used to transmit THz radiation through openings thatare too small for effective transmission of an unguided wave. The wavetransmitted in this manner can be received at a remote receiver or by areceiver mounted on the endoscope.

In other applications, the waveguide itself can serve as the samplingcontainer. In one such embodiment, illustrated in FIG. 7, the outer wall48 of the waveguide may be perforated with one or more openings 70 so asto allow the passage of a fluid (liquid or gas) into or through thewaveguide, as indicated by the arrows. If desired, two or more plugs 60can be used to define an inner chamber 72 and limit the axial flow offluid through the waveguide. In alternative embodiments (not shown),plugs 60 are provided with holes and allow fluid flow along the lengthof the waveguide, in addition to, or instead of the transverse flowillustrated in FIG. 6.

Probe or Sensor System

FIG. 8 shows a THz probe or sensor system comprising a first waveguide110 and a second waveguide 112. Waveguides 110, 112 may be coaxial,uniaxial, dual, or dual coaxial waveguides. First waveguide 110 may beused to transmit a THz signal, such as may be provided by a coupledcoaxial antenna, to a desired object of interest 120. Second waveguide112 receives the echo/reflected portion of the signal and transmits itto a receiver (not shown). The received signal contains informationabout the object of interest 120. A system such as this can be used tofacilitate inspection of enclosed spaces or other regions, or of objectsthat cannot be viewed using line-of-sight techniques or opticalinspection systems.

The system shown schematically in FIG. 8 may, in some embodiments, becombined with a reflector 114, as shown in FIG. 8. Reflector 130reflects the incident signal from waveguide 110 off an adjacent surfaceor object 122, from which the signal returns and is reflected intowaveguide 112, as indicated by the arrows in FIG. 9. This combinationallows inspection of a confined area of interest and is thereforeparticularly useful for endoscopy. Among other applications, THzendoscopy is proving particularly useful for cancer detection. Recentstudies have shown that THz is very useful for skin cancer detection,and it is being investigated for detection of cancers of the colon,esophagus, etc.

THz Apertureless Near-Field Scanning Optical Microscopy

According to another embodiment of the invention, a THz waveguide isused as the emitting tip of a near-field scanning optical microscope(NSOM). In this technique, light is scattered off a subwavelength-sizedmetal tip which is held close to a surface. The scattered light iscollected in the far-field, giving subwavelength resolution in theimmediate neighborhood of the tip apex. One embodiment of a THz NSOMsystem is illustrated in FIG. 10; in other embodiments the waveguidecould be coupled to a coaxial THz antenna.

Spectral Analysis

A still further embodiment of the invention is achieved when thedisclosed waveguides (e.g., uniaxial, dual, dual coaxial) includes aplurality of variations in the diameter of the core or conductiveelements 301, 303, as shown in FIG. 11. These form ridges 140, whichfunction as a diffraction grating, causing the THz signal to spread intoits spectral components and thus allowing filtering of the signal.

The desired diffraction can be achieved using a variety of techniques.For example, instead of varying the diameter of the metal wire, theexterior surface of the wire with can be coated with another material.The coating may vary in thickness, or may be periodic along the lengthof the antenna.

EXAMPLE 1 Propagation Effects in Near-Field Optical Antennas

The propagation of THz radiation along bare metal wires was firstobserved in the demonstration of apertureless near-field scanningoptical microscopy (NSOM) using THz-TDS. The experimental setup isdepicted in FIG. 12. The broadband single-cycle pulses of free-space THzradiation were generated using a photoconductive transmitter and werefocused onto a beryllium-copper probe acting as an aperturelessnear-field optical antenna. The probe had a tip of about 25 μm radiusand a shaft of 0.5 mm diameter coated with a thin layer of tin toprevent oxidation. The sample was a featureless gold-coated siliconwafer, placed in close vicinity of the tip. The mean distance betweenthe tip and the gold surface, d˜350 nm, was precisely controlled by apiezoelectric transducer. In such a configuration, the tip stronglyinteracted with its image in the metal surface, and converted thelocalized evanescent field around the tip to propagating radiationthrough a scattering process. The electric field of the scattered THzpulses was detected in the time-domain by a photoconductive receiverwhich is located near the tip. In the measurement, the probe tipvibrated normally to the surface at 750 Hz to modulate the scatteredradiation, and the detected signal is demodulated using a lock-inamplifier. The propagation of THz pulses was observed by moving theincident focal spot up along the shaft of the probe, as shown by thedashed arrow. In this case, some of the incident THz radiation wascoupled into a propagating mode on the shaft. This propagating modemoved down the shaft and excited the tip, producing a scattered wavewhich was detected by the receiver. Different incident positions lead todifferent propagation times and therefore different time delays in thedetected time-domain waveforms. A piece of metal perpendicular to theneedle was placed close to the shaft at the incident spot to provide asharp start point of the propagation. Scattering of the THz radiation atthe edge of this metal helped to couple the incident wave into apropagating mode on the shaft. The THz transmitter, the focusing lenses,and this metal scatterer were all mounted on a movable stage so that theincident position along the shaft could be precisely controlled.

The propagation effect was evident from the relative delay of thewaveforms obtained by moving the transmitter stage along the shaft ofthe needle in steps of 1.5 mm. As the point of incidence moved away fromthe tip, the pulse took longer to propagate along the shaft, and itsamplitude decreased. The propagation was largely nondispersive, sincethe shape of the time-domain waveform did not depend strongly onpropagation distance. The group velocity of the propagation mode couldbe extracted from the time-domain waveforms. The relative time delay ofthese waveforms showed a linear dependence on the propagation distance,as depicted by the squares in FIG. 12. A least-squares fit to these datayielded the group velocity v_(g)=(3.00±0.01)×10⁸ m/s, the free-spacevelocity of light. A similar measurement was performed in which theneedle is wrapped with a 0.5 mm PVC layer. The existence of theinsulator layer distorted the detected waveforms, and also reduces thegroup velocity of the propagation to 0.8 c, as depicted by the trianglesin FIG. 12. This result indicated that the propagation was confined andguided along the surface of the probe.

Besides the measurements with a bare needle and an insulated needle,propagation measurements were taken with a circular aluminum barriersituated on the probe, the disturbance and the reflection of thepropagation mode were then observed. These results revealed thepossibility of a new method for THz wave guiding and manipulating.However, the waveforms detected in these experiments were not theelectric field of the propagating THz pulses, but the scatteredradiation from the probe tip. To eliminate the spectral filteringeffects introduced by the probe tip, a new experimental configurationfor direct measurement of the THz propagation on bare metal wires wasrequired. This new configuration permitted us to fully characterize thepropagating mode along the wire waveguide.

EXAMPLE 2 Direct Characterization of a THz Wire Waveguide

For a better observation and characterization of the guided THzpropagation on metal wires, we changed the experimental setup from theNSOM configuration to a new configuration in which the electric field ofthe guided mode was directly detected at the end of the waveguide. Withthe fiber-coupled transmitter and receiver, the incident position (thestart point of the propagation) and the detection position of the THzpulses were changed to observe the spatial profile of the guided mode. Along stainless steel wire with a smooth surface, rather than the tinytapered probe in the NSOM experiments, was used as the waveguide for thenew measurements.

Experimental Setup

A schematic illustration of the new experimental setup is shown in FIG.13. As in the NSOM experiments, the broadband single-cycle pulses offree-space THz radiation were generated and coherently detected usingultrafast photoconductive sampling. The horizontally polarized THzpulses were focused onto the stainless steel waveguide. Anotherstainless steel wire was placed at the focal spot, orientedperpendicular to the waveguide (the y direction in FIG. 13). This secondwire served as an input coupler. As such, it could be replaced with afill or partial foil or mesh screen, or any other device capable ofcoupling the mode of the radiation to the mode of the waveguide.Scattering of the input THz radiation at the intersection structurehelped to excite the radially polarized mode which can propagate alongthe waveguide. Both the waveguide and the coupler are 0.9 mm indiameter, and the separation between them is 0.5 mm. The receiver wasplaced at the end of the waveguide and was oriented to detect only thevertically polarized component of the electric field in order toeliminate the possibility of detecting directly scattered radiationwhich would interfere with the detection of the guided mode. Theincident THz beam was modulated by a chopper in front of the transmitterand a lock-in amplifier was used for detecting the induced photocurrentin the receiver. The THz transmitter, the focusing lenses, and thecoupler were all mounted on a movable stage so that the incidentposition along the waveguide could be controlled. The THz receiver wasmounted on a three-axis stage for detection at various positions withrespect to the end of the waveguide.

Spatial Profile

As the first step in characterizing the propagating mode on the wirewaveguide, we measured the spatial profile of the electric field aroundthe waveguide by vertically scanning the THz receiver at the end of thewaveguide. FIG. 14( a) shows typical time-domain electric fieldwaveforms, for two different receiver positions located 3 mm above and 3mm below the wire waveguide. These waves are vertically (y) polarized,perpendicular to the horizontally (x) polarized input beam. The polarityreversal as the detector scans across the wire clearly shows the radialnature of the guided mode. The peak-to-peak amplitude of the waveform asa function of the vertical displacement of the receiver is depicted bythe squares in FIG. 14( b). The amplitude decreases with the transversedisplacement approximately as 1/r. Since the polarization response ofthe photoconductive receiver antenna is not perfectly symmetric, themeasured electric field is not precisely zero at the center point in theexperiment. This can also explain the slight asymmetry in the amplitudeprofile of the detected waveforms.

The observed behavior can be understood in terms of either the TEM modeof a coaxial waveguide or in terms of a Sommerfeld wave. The TEM mode ina coaxial waveguide is radially polarized, and the electric field variesas the inverse of the radial position, as

$\begin{matrix}{E_{r} = \frac{V}{r\; \ln_{b}^{a}}} & (1)\end{matrix}$

where a and b are the radii of the outer and inner conductors,respectively, and V is a position-independent voltage. Althoughproviding a qualitative picture, this description cannot be extended tocover the case of interest here because this expression vanishes in therelevant limit, a→∞. A more accurate picture can be obtained fromSommerfeld's description of an electromagnetic wave propagating alongthe surface of a cylindrical conductor, a so-called Sommerfeld wirewave. In this case, it has been shown that the important propagatingsolution is an axially symmetric TM wave. Outside the metal, thevariation of the radial electric field component (the dominantcomponent) is described by a Hankel function, H₁ ⁽¹⁾(γr), where γ isdefined in terms of the propagation constant k of the field outside thewire according to γ²=ω²/c²−k². For a perfectly conducting wire, γ=0 andthe field propagates with a velocity determined solely by the externalmedium (in our case, air). For large but finite conductivity, γ is smalland the approximate form for the Hankel function can be used,appropriate for small argument:

H₁ ⁽¹⁾(x)≈−2i/πx   (2)

Thus, a Sommerfeld wire wave also exhibits 1/r decay, within a distancer₀<<|1/γ| of the wire surface.

The Sommerfeld description can be used to estimate the distance that thewave extends from the metal surface, for a metal of finite conductivity.To do so, one must determine γ by solving the transcendental equationwhich results from the boundary conditions at the wire surface.Following the method described by Goubau, the amplitude of the wave wascomputed as a function of radial distance, for the case of a 0.9mm-diameter stainless steel (type 304) cylinder, with a conductivity of1.39×10⁶ mho/m, about 2.4% of the conductivity of copper. To account forthe finite aperture of our detector, we convolve this Hankel functionwith a Gaussian of 6 mm full-width at half-maximum. The resultingprofile is shown as a solid curve in FIG. 13( b). We can also calculatethe radius inside of which 50% of the power is guided. At a frequency of0.3 THz, half of the power is transmitted through an area extendingroughly 1.2 millimeters from the axis of the wire. The reasonably goodagreement between the experimental results and the calculations suggeststhat the surface wave picture is an appropriate model for ourexperimental situation. However, as discussed below, thefrequency-dependent attenuation is still lacking a quantitativedescription.

Propagation Characteristics

The propagation characteristics of the guided mode were studied bymoving the incident position of the THz beam along the waveguide. Inthis way, the time-domain waveforms as a function of propagationdistance were obtained. There was no evident change in the temporalshape of the waveforms for propagation up to 24 cm, the limit of ouroptical delay line. This showed that the propagation was largelydispersionless. As in the NSOM experiment, the broadband group velocityof the propagation mode was determined by analyzing the dependence ofthe relative time delay of the waveforms on the propagation distance. Aleast-squares linear fit to these data yields the group velocityv_(g)=(2.995±0.001)×10⁸ m/s, as shown in FIG. 15( a). FIG. 15( a) showedthe arrival time as a function of incident position for a series oftime-domain THz pulses detected using the setup illustrated in FIG. 13(squares). The spectrum-weighted average group velocity of the guidedmode was obtained from the least-squares linear fit to these data. TheTHz waveforms detected after 4 cm and 24 cm of propagation are shown inthe insets. To study the group velocity dispersion, the group velocityfor different frequency components was extracted by analyzing thespectra of these waveforms, using

$\begin{matrix}{v_{g} = \frac{c}{{n_{eff}(\omega)} + {\omega \frac{n_{eff}}{\omega}}}} & (3)\end{matrix}$

where n_(eff) is defined as

$\begin{matrix}{{n_{eff}(\omega)} = {{{\Delta\varphi}(\omega)}\frac{c}{\omega \; d}}} & (4)\end{matrix}$

Δφ(ω) is the phase change for propagation distance d at angularfrequency ω. FIG. 15( b) shows the extracted data, confining that thereis no measurable group velocity dispersion throughout the accessiblespectral range. This was to be expected, given that the Sommerfeldsurface wave model predicts a group velocity deviating from c by lessthan one part in 10⁴, for our experimental situation.

In order to study the evolution of the guided mode in propagation, wecompared the spatial profile of the guided mode detected at differentpropagation distances, each obtained in the same manner as that in FIG.14( b). These profiles are depicted by the curves in FIG. 16. It isimmediately clear that the electric field is more closely confined tothe surface of the wire for the shortest propagation distances.Subsequently, the guided mode spreads laterally, especially during thefirst several centimeters of propagation, and approaches a spatialprofile described roughly by 1/r.

For each propagation distance, the waveform with the maximumpeak-to-peak amplitude was extracted. Except for the few shortestpropagation distances, these were obtained at a fixed receiver offset ofroughly 3 mm (see FIG. 16). These amplitudes were plotted as a functionof propagation distance in FIG. 17( a). The amplitude attenuationcoefficient α of the wire waveguide can be extracted from these data,simply by fitting the dependence of the pulse amplitude E on thepropagation distance x to:

E(x)=E₀e^(−αx)   (5)

The value we obtained, α=0.03 cm⁻¹, was the lowest of any waveguide forbroadband THz pulses reported to date. This method gave us thespectrum-weighted amplitude attenuation coefficient, but a more detailedcharacterization was required to obtain the frequency dependence of theloss. We extracted the attenuation coefficient of each frequencycomponent from the amplitude spectra of the THz waveforms detected atvarious propagation distances. The spectrum of the attenuationcoefficient is shown in FIG. 17( b). We note that the attenuationdecreased with increasing frequency.

The low attenuation obtained here emphasized one unique aspect of thewire waveguide. Compared to other waveguide geometries, a metal wire hada much smaller surface area interacting with the electromagnetic field,so the propagation loss due to finite conductivity of the metal wasnegligible. This is consistent with Sommerfeld's wire wave model, whichpredicted a very small propagation loss due to the finite conductivityof the metal wire. However, the spectrum of the attenuation obtained inour experiment can not be described simply by the Sommerfeld model, asshown in FIG. 17( b). The predicted losses increased with increasingfrequency, similar to other THz waveguides where the attenuation isdominated by ohmic effects. In contrast, the observed losses decreasewith increasing frequency. This indicated that much of the measuredlosses arise from other sources, such as diffractive spreading of thepropagating mode in the lateral dimensions, as seen in FIG. 16. Thesignificance of this loss mechanism for Sommerfeld waves has beendiscussed previously. By moving the receiver away from the end of thewaveguide, we observe a sharper drop in the amplitude of the detectedpulses, as depicted by the hollow squares in FIG. 17( a), indicating anincreasing divergence when the mode propagates off the end of thewaveguide.

The measurements do not reflect the losses associated with the couplingof the linearly polarized free-space THz beam to the guided mode. In theexperiment described here, only about one percent of the power iscoupled to the radially polarized waveguide mode from the free-spaceincident beam. The more effective mode-matching needed to improve theinput coupling can be obtained using the mode-matched antennas describedabove.

EXAMPLE 3 Manipulation of the Guided Pulses

The manipulation of the guided mode was next studied. The ability todirect radiation along curves is one of the most important features fora practical waveguide. The amplitude of THz pulses was compared afterpropagating on a waveguide bent with different radii. The results areshown by the hollow triangles in FIG. 18. The propagation distance was21 cm, and the radius of curvature R was varied from 90 cm down to 20 cmin steps of 10 cm. The amplitude of the electric field E′ as a functionof the propagation distance x along the bent waveguide is described by

E′(x)=E ₀ e ^(−α′x)   (6)

where α′ is the amplitude attenuation coefficient for a bent waveguide.By comparing equation (6) to equation (5) we find

$\begin{matrix}{\alpha^{\prime} = {\alpha + \frac{\ln \left( \frac{E}{E^{\prime}} \right)}{x}}} & (7)\end{matrix}$

So the amplitude attenuation coefficient for each bend radius wasextracted by comparing the amplitude of the detected THz pulse to thatof a straight waveguide with the same propagation distance x. Theextracted data are depicted by solid squares in FIG. 18. Even a slightbend on the waveguide lead to a considerable increase in the loss, from0.03 cm⁻¹ for a straight waveguide to nearly 0.05 cm⁻¹ for a bend radiusof 90 cm.

The bend loss can be explained by the continuous conversion of theguided propagation into radiation modes as the wave travels around acurve. This is easy to understand by considering the wavefront of thetransverse field, which must rotate around the center of the curvatureduring propagation. Consequently, at some distance from the center ofcurvature the phase velocity would exceed c, the propagation speed ofthe guided mode. So the portion of the field outside this point must beradiated, causing the power loss in the guided mode. This loss mechanismresembles that of a bent dielectric optical waveguide, in which theattenuation coefficient α can be described by a semi-empirical form:

α=c ₁ exp (−c ₂ R)   (8)

where R is the radius of curvature and c₁ and c₂ are constantsindependent of R. A fit using equation (8) shows a good agreement withthe experimental data, as seen in FIG. 18, suggesting that radiation isthe dominant mechanism for the propagation loss of a bent metal wirewaveguide.

From the spatial profile of the propagation (FIG. 14( b) and FIG. 16) wecan see that the guided mode has a large extent compared to the crosssection of the waveguide. Hence we predict that the guided mode would beeasily coupled between two curved waveguides in contact with each other(or between a curved waveguide and a straight one). These featuresenable the Y-splitter for the wire waveguide, as illustrated in FIG. 19.The validity of this scheme has been verified by electric fieldmeasurements with such a structure. The waveforms in FIG. 20 aredetected at points A, B, and C in FIG. 19, namely: at the end of thestraight waveguide (A), at the end of the curved waveguide (B), and at aposition between them (C). The separation between A and B is 2 cm, andthe waveforms are detected with the THz receiver 3 mm below the plane ofthe splitter structure. The plot clearly shows that part of thepropagating power on the straight waveguide is coupled to the branchwaveguide by the Y-splitter.

Besides the waveguide described above, we have also tried many othermetal wires as THz waveguides. The materials for these guides includesteel, aluminum, copper, zinc and nickel-chrome alloys. The wirediameter of these guides ranges from 0.5 mm to 6.4 mm. In situationswhere the guided mode could be perturbed by other structures close tothe waveguide, an outer metallic shield could be provided, forming acoaxial waveguide, as long as the additional ohmic losses could betolerated.

With a Y-splitter structure used to separate the output wave from theinput wave, and a small mirror attached at the end of the waveguide as a90-degree output director, we have successfully demonstrated a THzendoscope, by detecting THz pulses reflected from the bottom and theside wall inside a container. Further improvement could be made bycombining an endoscope with an imaging system. This may be accomplishedby scanning the endoscope along the surface of the detected region, oralternatively, scanning or rotating the sample to obtain an internal THzimage. One challenge for this application is the low power transmittedby the endoscope which strongly limits the data acquisition rate as wellas the dynamic range. With optimization of the mode of the input beamand the coupling geometry using the invention described above, the powerlaunched into the endoscope probe can be greatly increased.

It is also interesting to note that this waveguide naturally generates aradially polarized mode. So with a focusing lens mounted at the distalend of an endoscope, a higher resolution can be obtained than in thenormal THz imaging system, due to the sub-diffraction-limited focusingof radially polarized beams. Furthermore, since the radially polarizedmode is an ideal input field for a coaxial near-field probe or anapertureless near-field optical antenna, nanometer-resolved endoscopicTHz imaging may be possible. This would pave the way for a wide range ofnew applications for terahertz sensing and imaging.

While embodiments of the invention have been shown and described,modifications thereof can be made by one skilled in the art withoutdeparting from the spirit and teachings of the invention. Theembodiments described and the examples provided herein are exemplaryonly, and are not intended to be limiting. Many variations andmodifications of the invention disclosed herein are possible and arewithin the scope of the invention. Accordingly, the scope of protectionis not limited by the description set out above, but is only limited bythe claims which follow, that scope including all equivalents of thesubject matter of the claims.

The discussion of a reference in the Description of the Related Art isnot an admission that it is prior art to the invention, especially anyreference that may have a publication date after the priority date ofthis application. The disclosures of all patents, patent applications,and publications cited herein are hereby incorporated herein byreference in their entirety, to the extent that they provide exemplary,procedural, or other details supplementary to those set forth herein.

REFERENCES

The following references provide background information and are eachincorporated herein by reference in their entireties, except to theextent that they define terms differently than those terms are definedherein:

1. Miyaji, G; Miyanaga, N; Tsubakimoto, K; et al. Intense LongitudinalElectric Fields Generated From Transverse Electromagnetic Waves APPLPHYS LETT, 84 (19): 3855-3857 May 10, 2004

2. Dorn, R; Quabis, S Leuchs, G Sharper Focus For A Radially PolarizedLight Beam PHYS REV LETT, 91 (23): art. no.-233901 Dec. 5, 2003

3. Armstrong, D J; Phillips, M C; Smith, A V Generation Of RadiallyPolarized Beams With An Image-Rotating Resonator APPL OPTICS, 42 (18):3550-3554 Jun. 20, 2003

4. Moshe, I; Jackel, S; Meir, A Production Of Radially Or AzimuthallyPolarized Beams In Solid-State Lasers And The Elimination Of ThermallyInduced Birefringence Effects OPT LETT, 28 (10): 807-809 May 15, 2003

5. Grosjean, T; Courjon, D; Spajer, M An All-Fiber Device For GeneratingRadially And Other Polarized Light Beams OPT COMMUN, 203 (1-2): 1-5 Mar.1, 2002

6. Bomzon, Z; Keiner, V; Hasman, E Formation Of Radially And AzimuthallyPolarized Light Using Space-Variant Subwavelength Metal Stripe GratingsAPPL PHYS LETT, 79 (11): 1587-1589 Sep. 10, 2001

7. Oron, R; Blit, S; Davidson, N; et al. The Formation Of Laser BeamsWith Pure Azimuthal Or Radial Polarization APPL PHYS LETT, 77 (21):3322-3324 Nov. 20, 2000

1. A terahertz (THz) transmitting system comprising: an antenna capableof generating a beam of THz radiation; and a waveguide aligned with saidbeam, said waveguide having at least a first and a second conductiveelongate element, wherein said first and second conductive elongateelements are substantially parallel.
 2. The terahertz transmittingsystem of claim 1, wherein said at least first and second conductiveelements comprise a conductive metal.
 3. The terahertz transmittingsystem of claim 1, wherein said antenna generates a beam of linearlypolarized THz radiation.
 4. The terahertz transmitting system of claim1, wherein said antenna comprises a linear dipole photoconductive THzantenna.
 5. The terahertz transmitting system of claim 4, wherein saidantenna generates a beam of radially polarized THz radiation.
 6. Theterahertz transmitting system of claim 1, wherein said waveguidecomprises an outer wall, wherein said outer wall is impermeable to THzradiation.
 7. The terahertz transmitting system of claim 1, wherein saidfirst and second conductive elements form a bend.
 8. The terahertztransmitting system of claim 1, wherein said first and second conductiveelements are substantially uniformly spaced along their lengths.
 9. Theterahertz transmitting system of claim 1, further comprising acollimator disposed in between said antenna and said waveguide to focussaid beam of THz radiation.
 10. The terahertz transmitting system ofclaim 1, wherein said first and second conductive elements are wires.11. The terahertz transmitting system of claim 1, wherein said a firstand second conductive elements each have a circular cross section. 12.The terahertz transmitting system of claim 1, wherein said first andsecond conductive elements diverge.
 13. The terahertz transmittingsystem of claim 1, wherein said waveguide comprises more than twoconductive elements.
 14. A method for transmitting a terahertz signalcomprising: a) aligning an antenna capable of generating a beam of THzradiation with a waveguide comprising at least a first and secondconductive element; b) generating a beam of THz radiation by focusing alaser at said antenna; c) collimating and focusing said beam between thefirst and second conductive element of said waveguide; and d)transmitting said beam along the length of said waveguide by propagatingsaid beam between the first and second conductive elements.
 15. Themethod of claim 14, wherein said antenna comprises a linear dipolephotoconductive THz antenna.
 16. The method of claim 14, wherein (b)comprises generating a beam of linearly polarized THz radiation.
 17. Themethod of claim 16, wherein said laser is an ultrashort pulsed laserbeam.
 18. The method of claim 14, wherein the first and secondconductive elements diverge to form a first waveguide and secondwaveguide.
 19. The method of claim 18, further comprising splitting thelinearly polarized beam into a first and a second radially polarizedbeam, wherein the first radially polarized beam propagates down thefirst waveguide and the second radially polarized beam propagates downthe second waveguide.
 20. The method of claim 14, wherein (b) comprisesgenerating a beam of radially polarized THz radiation.
 21. A terahertzinterferometer comprising: a THz emitter that generates THz radiation; aprimary waveguide aligned with said emitter, said waveguide comprises atleast a first and second conductive element, wherein said at least firstand second conductive element diverge to form a sample waveguide and areference waveguide; a sample chamber, wherein said sample waveguidepasses through said sample chamber, wherein said sample waveguide andsaid reference waveguide rejoin to form a secondary waveguide; and adetector capable of receiving THz radiation, said detector aligned withsaid secondary waveguide.
 22. The terahertz interferometer of claim 21,wherein said THz emitter comprises a linear dipole THz antenna.
 23. Theterahertz interferometer of claim 21, wherein said detector comprises alinear dipole THz antenna.
 24. The terahertz interferometer of claim 21,wherein said reference waveguide passes through a reference chamber. 25.The terahertz interferometer of claim 21, wherein said sample chambercontains a solid, a liquid, or a gas.
 26. The terahertz interferometerof claim 24, wherein said reference chamber contains a solid, a liquid,or a gas.